1.) Field of the Invention
This invention generally relates to light beam scanning systems and methods of scanning printed indicia such as bar code symbols and other optically discernible materials from which it is desired to extract information.
2.) Description of Prior Art
In a beam scanner a small spot of light is focused upon and swept rapidly across a target from which information is to be read. A bar code reader is an important commercial application of a light beam scanner and is referred to herein as a typical example of a specific application for the present invention. In a bar code reader the light is usually provided by a laser diode emitting red light. After reflection from the target, light is collected onto a photo electric converter such as a photo diode whereupon the collected light is converted into electronic signals representing features of the target.
Millions of portable commercial laser bar code scanners and contact type bar code reading wands presently in service operate at scan speeds on the order of 40 scans per second or less. The large number of installed decoders used with these are only compatible with scanning rates in this speed range. Portable laser scanners like those described in U.S. Pat. No. 4,387,297 typically resemble a personal hair dryer in size and shape. Although these scanners automatically scan a light beam, they must be carefully aimed at bar code targets to be read, and must be positioned within the proper distance range from the target where the beam is sufficiently focused to resolve features of the target. A manual trigger is then pulled with the index finger of the user and a scan attempt is made. The scan attempt can last for up to a second and during this time small adjustments in aiming and range can be made before the read attempt is automatically terminated by the device. Inexperienced users may try to pull the trigger and sweep the scanner past a target hoping to catch it in order to avoid aiming. However, due to relatively slow start up times and the slow scan rates of presently available hand held scanners, the scan line may be too narrow or may simply zigzag above and below the target, missing it entirely. Even experienced users depend on visual feedback to see that the scan line is properly intersecting a bar code target, but no visual feedback on spot focus is available to the user of present scanners. Therefore trial and error is the only way an operator may successfully position the scanner away from the target. In bright lighting conditions the red scan line is difficult to see, and color blind users may not be able to see the red scan line at all. If scan speeds were increased beyond 40 scans per second the scan line would become even more difficult to see. Also, for dense bar codes having narrow lines and spaces, the useful depth of field for most portable laser scanners is only a few inches. (For 6 mil bars and spaces this distance is typically about three inches.) Focus mechanisms similar to those used in automatic cameras might be designed for beam focusing, but these are quite complex and slow and thus have not been successfully commercialized. Some attempts have been made to adapt the hand holdable gun shaped scanners to fixed mount point of sale applications by designing stands for them whereby they are aimed at a counter top area and operated in a continuous scan mode. These attempts have not worked well because even greater skill is needed for an operator to align and position the goods to be read. This usually proves more difficult and cumbersome than manually positioning the scanner. These examples highlight the long standing need for a small scanner which requires little orientation with respect to the target being read and one which is equally suited to both fixed or hand held operation in existing systems. The contact reading wands, which do not scan a beam, must be held in contact with the bar code and manually drawn across it by the user; they are notoriously poor at reading non-flat surfaces, and repeated attempts to read a bar code are often needed. The advantages of a miniature high speed non-contact bar code scanner which may be held with the fingers like a pen were pointed out in U.S. patent application 08/114,646 of which the present application is a continuation-in-part. In that application, miniature beam scan components, hereafter referred to as "scan elements", for a pen size beam scanner are disclosed. Several of the miniature scan elements disclosed therein are capable of scan rates in the range of hundreds to thousands of scans per second. These require special high speed decoders and are not compatible with many of the already installed low speed decoders. Also disclosed are beam scanner embodiments which are low speed but which require delicate torsion springs to achieve low scan speeds.
In designing scan elements of reduced size, it becomes increasingly difficult to produce scan rates which are relatively slow, on the order of 40 scans per second. This is because as the moving components of the scanners such as mirrors, magnets and coils are reduced in size, their inertia is also reduced. Furthermore, a restoring force is usually required for the oscillating scan components and is often supplied by metal springs, torsion elements or the like; these devices, when reduced in size and operated with extremely small inertial loads, cause the resonant frequencies of the scanner to be high, thus tending to produce high scan rates.
Achieving adequate scan angles on the order of 40 degrees also becomes a problem in the design of miniature scanners with mechanical springs or torsion elements. This is so because as the length of a spring element is shortened, so is its range of travel, whereas the elastic limits of many materials used for springs may be approached well before they flex enough to produce sufficient deflection to achieve an adequate scan angle. Operation of such springs close to their elastic limit will lead to fatigue and early failure.
If one attempts to use an extremely thin spring material with a very low force or torsion constant to fabricate a small low frequency scanner, it has been found that such a scanner may be very fragile and susceptible to picking up unwanted external vibrations especially in portable hand held applications.
The present invention incorporates many principles disclosed in the prior patent applications of which the instant application is a continuation-in-part and discloses new embodiments which are miniature in size yet capable of a wide range of scan rates from a very low scan rate of less than ten scans per second to moderately high rates of over one hundred scans per second. At the same time these devices are rugged and can easily achieve scan angles on the order of sixty degrees if necessary. In addition, the depth of operating range is extended with a novel focal system which is integrated with the light source, and special positioning and automatic triggering techniques are disclosed which make the present invention more versatile and equally suited to both fixed mount and hand held operation.